1. Field of the Invention
The invention relates to a metering unit and a method for metering liquid or gaseous educts for a fuel cell system.
2. Prior Art
Among the alternative drive concepts for motor vehicles, fuel cell-supported systems are gaining increased attention at present. These systems typically contain PEM (polymer electrolyte membrane) fuel cells, which are operated with hydrogen and air as energy vehicles.
Since as before, it still proves problematic to put hydrogen in a tank and store it in the motor vehicle, the hydrogen is produced in an upstream reformer stage, from easily handled fuels such as methanol, methane, Diesel or gasoline, directly “onboard” as needed and is consumed immediately. In such fuel cell systems, many flows of material must accordingly be metered flexibly and nevertheless quite precisely. This is true both for liquid components, such as superpure water, fuels and coolant, and for such gaseous media as air or methane. The primary problem in the metering is that pressure fluctuations in the transport lines make exact metering of the individual components more difficult. These pressure fluctuations can be caused on the one hand by upstream pumps or compressors, but also by the chemical reactions that take place in the reformer, for instance, and can release process gases and thus lead to pressure return shocks.
In German Patent DE 44 25 634 C1, a method and an apparatus for metering liquids for a fuel cell system are described; the metering is done via the cycle time of a magnet valve, and the pressure difference between the feed line and the fuel cell system is regulated via a differential pressure valve. This arrangement, because clocked switching valves are used, creates fluctuations in the volumetric flow in the transport lines, which can cause problems in the chemical reactions that take place in the reactors of the fuel cell system.
In motor vehicles, fuel injection systems are used, which make it possible to meter the fuel at different pressure conditions. These systems, known by the trademark K-Jetronic, contain a combination of a control piston and a differential pressure valve. These are air-pressure-controlled systems, whose use is limited to the metering of fuels.
The object of the present invention is to furnish a metering unit for liquid and gaseous components for a fuel cell system. Pressure fluctuations inside the transport lines are compensated for, and exact metering is made possible.